Silicon carbide carrier for wafer processing and method for making same

ABSTRACT

A single piece, high purity, full density semiconductor wafer holding fixture for holding a multiplicity of wafers and consisting essentially of chemical vapor deposited silicon carbide (CVD SiC). The wafer carrier is advantageous for the fabrication of electronic integrated circuits where high temperatures and/or corrosive chemicals present, where dimensional stability of the holder is advantageous to the process or where introduction of contaminating elements is deleterious to the process. The method for making such an article comprises shaping a substrate, e.g. graphite, which on one surface has the form of the desired shape, said form comprising raised longitudinal sections to support the silicon wafers at the edges of the wafers, chemically vapor depositing a layer of silicon carbide onto the substrate, removing the substrate intact or by burning, machining, grinding, gritblasting and/or dissolving, and grinding the silicon carbide in any areas where a more precise dimension is required.

This is a division of application Ser. No. 08/286,942 filed Aug. 8,1994, now U.S. Pat. No. 5,538,230.

FIELD OF THE INVENTION

The invention relates to fixtures or carriers ("boats") used in themanufacture of semiconductor devices such as diodes, transistors andintegrated circuits. More particularly, the invention relates to siliconcarbide carriers that are used in semiconductor fabrication processesthat require the use of high temperatures and/or corrosive fluids. Thecarriers are used in chemical or thermal processing of semiconductorwafers, in horizontal processing equipment, at ambient or elevatedtemperatures, in vacuum, gaseous or liquid processing environments.

BACKGROUND OF THE INVENTION

The requirements for cleanliness and the elimination of contaminants inthe processing of semiconductor wafers are well documented; see, forexample U.S. Pat. Nos. 3,951,587, 3,962,391, 4,093,201, 4,203,940,4,761,134, 4,978,567, 4,987,016. To maintain extremely high purityduring processing, it is known that such fixtures should be totally freeof contaminants, to the extent commercially feasible. It is also knownthat the carriers should be stable at elevated temperatures, and whensubjected to corrosive or oxidizing conditions. Typical corrosive oroxidizing conditions to which the carriers should remain inert are setforth in U.S. Pat. Nos. 4,987,016 and 4,761,134.

Quartz has been and continues to be the most common material used forthese components and fixtures. However, quartz has certain deficiencies,such as structural weakness at high temperatures, susceptibility toetching by commonly used acids, and a coefficient of thermal expansionthat differs from that of certain materials which are deposited thereonduring normal use.

The prior art discussed below addresses the construction of siliconcarbide (SiC) boats which have thus far had the most commercialsuccess--a porous SiC formed by casting. These references disclose thedrawbacks of quartz, and the benefits of using SiC. They also disclosethe drawbacks of the casting method of producing SiC boats. In order toavoid the deficiencies of porous SiC, prior art methods apply a SiCcoating layer by chemical vapor deposition (CVD) over the cast SiC.However, an overcoat of CVD SiC does not completely eliminate theproblems with porous SiC, since the coating can crack or chip, andthereby expose the porous SiC. Thus, a carrier that consists entirely ofCVD SiC is preferred, thus avoiding the problems of porous SiC.

A number of attempts have been made to improve on quartz. The mostsuccessful is a porous silicon carbide infiltrated with silicon,disclosed in U.S. Pat. No. 3,951,587. The problem with such a carrier isthat the silicon can etch out when exposed to commonly used cleaningsolutions, e.g., strong acids, such as nitric acid, illustrated in U.S.Pat. No. 4,761,134. Other workers (see U.S. Pat. No. 4,761,134) proposeto solve this problem by applying an impervious coating, generally achemical vapor deposited silicon carbide (CVD SiC) coating on thesurface of the silicon-filled silicon carbide, or on a porous siliconcarbide that has not been filled with silicon (see U.S. Pat. No.4,987,016). The drawback to these approaches is that any chip, break orcrack in the coating will expose the undesirable substrate. U.S. Pat.No. 4,761,134 discusses this drawback as it pertains to CVD SiC appliedon an unfilled porous SiC substrate. However, the discussion neglects topoint out that a similar weakness is inherent in the approach disclosedand claimed in this reference. U.S. Pat. No. 5,283,089 disclosesdepositing silicon carbide or silicon nitride onto a silicon carbide orsilicon nitride matrix to form wafer boats and other components forsemiconductor diffusion furnaces.

The preferred approach is to fabricate the carrier entirely from CVDSiC. In this approach, there is no possibility of a silicon-filled orporous substrate being exposed. The CVD SiC fixture also has theadvantage of being cleaner than the cast and sintered, orreaction-bonded SiC carriers disclosed in the previously citedreferences.

The following references describe carriers that are composed entirely ofSiC. U.S. Pat. No. 4,978,567 describes a CVD SiC fixture for processinga single wafer at a time, in a furnace designed to do single waferprocessing. There is a strong need for a boat for batch processing,capable of holding from 25 to 50 or more wafers. The carrier describedand claimed in U.S. Pat. No. 4,978,567 cannot be used for batchprocessing.

It is also known to use a hollow beam made of CVD SiC to hold wafers.However, this device requires that three or four such beams be joinedtogether by a means of support at the ends. While such a boat fulfillsthe need for a boat that can hold a plurality of wafers duringsemiconductor processing, the boat is fragile and relatively complex,and hence costly to manufacture. There is a need for a carrier that usesa single piece of SiC to achieve the same result, and thus is strongerand more economical.

Japanese Patent Application No. Sho 55-82427 discloses a boat consistingof a single piece of silicon carbide, formed by CVD on a graphitesubstrate. However, the the boat has a rectangular cross-section, whichis undesirable because it requires an inefficient use of furnace space.Moreover, the rectangular design causes the mass of the boat to beunnecessarily large, which adds excess thermal inertia, and distorts thethermal pattern developed in the wafers during processing. In diffusionprocesses, for example, the excess thermal mass of a boat can causetemperature variations across the wafer area, and thereby alterdiffusion patterns. Such variations cannot be offset by changes inprocess parameters. Still further, excess wafer area is covered by theslot walls or the connecting end members. In addition, the designincludes partially enclosed areas that will distort gas flow patterns,and will increase the time required to exaust gases contained in suchpartially enclosed areas.

JP 55-82427 also fails to reveal the size of the boat, relative to thesize of the wafers to be carried. If the height of the boat is smallrelative to the wafers, the slots will not provide adequate horizontalsupport to maintain the wafers in a vertical position. If the height ofthe boat is large relative to the diameter of the wafers, adequatehorizontal support will be provided, but the walls of the slots willthen cover an excessive area of each wafer.

U.S. Pat. Nos. 3,962,391, 4,093,201 and 4,203,940 (all assigned toSiemens) describe methods for making carriers which hold a number ofwafers and are produced by depositing CVD silicon or CVD silicon carbideon a generally cylindrical graphite form. However, these patentsdescribe carriers that are not suitable for the most widely used waferprocesses. The devices described in U.S. Pat. Nos. 3,962,391 and4,093,201 do not have means for holding the wafers apart, with a uniformgap between each of the wafers which is required in most batchsemiconductor processes. U.S. Pat. No. 4,203,940 describes a carrierdesign that requires the grinding of slots in a silicon or siliconcarbide form to provide means for holding the wafers apart with a gapbetween each pair of wafers. However, the design described in the latterpatent allows only two narrow slots to hold each wafer.

Since two slots do not provide adequate wafer support, the industry hasdeveloped carriers having four support points to hold each wafer. Thisis beneficial in the processing of silicon wafers, since it allows thewafers to be held in a more uniform and more parallel position, whileminimizing the wafer area covered by the support points. Minimizing thewafer area covered by the support points of the holder maximizes thearea of the wafer available for productive use. The general guidelinesfor the design of these widely used wafer carriers is described in theSEMI International Standards, published by Semiconductor Equipment andMaterials International, Mountain View, Calif. The information containedin these standards is incorporated herein by reference.

SUMMARY OF THE INVENTION

The present invention provides a horizontal CVD SiC wafer boat for usein the manufacture of semiconductor devices, including features thateliminate the disadvantages of the above-described structures, andhaving the added advantage of conforming with the dimensionalrequirements described generally in the aforementioned SEMI standards.

The preferred carrier of the present invention consists of a singlepiece of nonporous SiC having a uniform bulk density in excess of 3.18grams per cubic centimeter (99% of maximum theoretical density), and apurity of at least 99.99%. It is configured as a generally cylindricalshell section having an average inner radius slightly greater than theradius of the wafers to be held. The generally concave inner surface ofthe carrier includes at least two longitudinally uniform convex portionswherein a plurality of orthogonal slots or grooves are located, toprovide the necessary wafer support. Since the carrier is to be used ina horizontal position, each of the wafers is thereby supported in avertical position, parallel to each other. In a preferred embodiment,the carrier walls have a substantially uniform thickness, except for theareas where the wafer slots are located. The slot areas of the carrierwall are preferably about one-half to three-fourths as thick as theremaining walls. This feature minimizes thermal interaction between thewafers and the carrier; and also reduces the amount of wafer surfacecovered by the slot walls.

The convex inner portions of the carrier need not have a singlecurvature. Instead, when two elongated wafer support slots are desired,the convex inner surface portions are shaped to include flat or slightlyconcave central segments, such that the slots provided therein have auniformly shallow depth, whereby only a minimum area of the wafer edgesare covered, after insertion into the slots. In such a configuration thelower end of each slot provides vertical and horizontal support, whilethe upper end of each slot provides only horizontal support, oralignment, to ensure a uniform parallel separation between adjacentwafers.

Both ends of the cylindrical section can be left open to form anopen-ended carrier. Or, the ends can be closed to form a closed-endcarrier. The outer surface of the shell may include one or moreflattened sections to provide a stable base for the carrier. Or,parallel longitudinal external ridges may be provided on theoutersurface of the shell to permit the carrier to be supported duringprocessing by cantilever paddles, or other support means, or for liftingdevices. Open areas may also be formed in the bottom or in the sides ofthe carrier, for gas circulation, or the draining of fluids used incleaning or wet processing, or to reduce the mass of the carrier, or forother reasons, such as the insertion of a lifting device.

Another aspect of the invention is embodied in a process for making thecarrier, which begins with the step of shaping the exterior of asubstrate or mold, to provide the exact geometry required for the insidesurface of the carrier. A graphite mold, for example, is shaped into theform of a cylindrical section, machined to provide a longitudinallyelongated flattened area on its convex surface, and then furthermachined to provide one or more longitudinally uniform concave surfaceson each side of, and parallel to the flattened area. The mold is maskedin areas where no deposition is desired.

Next, silicon-carbide is formed on the mold surface, by chemical vapordeposition; and the resulting CVD shell is separated from the mold. Theflattened area of the mold provides the flat bottom of the carrier, andthe concave surfaces of the mold provide the convex inner portions ofthe shell. Orthogonal slots or grooves are machined into the convexinner portions, to provide wafer support points. Other features of theboat may also be shaped by grinding, such as the length and height, andthe width of the bottom and base widths. Separation of the mold isusually achieved by destructively burning away the graphite, wherebyonly the deposited shell remains. Such grinding of the SiC may beperformed before or after removing the mold, or a combination of beforeand after.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of one embodiment of the present invention,an opened end wafer carrier having a flattened bottom, and open areaswith a wafer shown in phantom.

FIG. 2 is an isometric view of another embodiment of the presentinvention, a closed end wafer carrier having a flattened bottom and openareas.

FIG. 3 is a cross-sectional view of another embodiment of the presentinvention, an open-ended wafer carrier having a flattened bottom and twoinwardly extending surfaces, each inwardly extending surface having acentral concave segment between two convex segments.

FIG. 4 is a cross sectional view taken along line 4--4 of the carriershown in FIG. 1, with a wafer shown in phantom.

FIG. 5 is a cross-sectional view of another embodiment of the invention,an open-ended boat wherein each wafer is supported by three slots.

FIG. 6 is a detailed view taken along line 6--6 showing four of theslots of the carrier shown in FIG. 1.

FIG. 7 is a cross sectional view taken through the longitudinal centerof a carrier of the present invention in the process of being formed andshows the SiC after deposition onto a form in which a mask is placedover the back of the form.

FIG. 8 is an end view of two open-ended carriers of the presentinvention in the process of being deposited on a cylindrical form beforethe carriers are removed from the form.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIGS. 1 and 4, boat 20 is shown comprising cylindricalshell section 22 having a thickness t, of chemical vapor deposited SiCand having outer convex surface 23, inner concave surface 24, open firstend 25 and open second end 26.

Cylindrical shell 22, which makes up the major portion of boat 20, liesoutside the radius of inner surface 24, r1, which is greater than theradius, r2, of wafer 27 to be supported. Cylindrical section 22 subtendsan arc, i.e. angle α, ranging from approximately 90° to approximately180°. The specific geometry of the portion of the boat lying outsideradius r1 may take many forms, but generally has flat base 30 forresting boat 20 on a horizontal surface. Base 30 has a width, w, whichmay serve to position the boat in a semiconductor processing furnaceand/or in devices for loading and unloading the wafers. Boat 20 shown inthis embodiment also includes inward protrusions 31 having radius r5,which allows one to mount lifting devices for moving boat 20. Inwardprotrusions 31 may also be used to support boat 20 on a paddle, e.g. aconventional U-shaped paddle, or other devices which supports boat 20 atpositions other than base 30. Cut outs 32 may be used to insert liftingdevices from the side. Other cut outs 33 in base 30 are used to permitthe flow of process gases as well as to decrease the weight of boat 20by removing unnecessary mass.

Boat 20 has four slot-containing inwardly convex surfaces 34 whichextend toward the wafer center to a distance r3. Surfaces 34 areprovided with a plurality of grooves or slots 35 into which the wafersare placed. The bottom or outermost radius, r4, of slots 35 is slightlylarger than the r2 of the wafers to be supported, as is typical of theslotting of wafer boats in the industry.

The length, L, of slots 35 is calculated by determining the angle, θ,subtended by the slots. In the carriers of this invention, θ ranges fromapproximately 5° to approximately 60°. L must increase and hence θ mustincrease if a wafer carrier has only two slot-containing inwardlyextending surfaces. In this case, each of the slots subtends an arc,i.e. angle θ, ranging from approximately 20° to approximately 60°.

FIG. 2 illustrates boat 36 which is identical to boat 20 shown in FIG. 1except that it has closed first end 37 and closed second end 38.

FIG. 3 illustrates boat 40 having only two inwardly extending surfaces34, each of which has a row of slots 35 equally spaced along thelongitudinal axis of the cylindrical shell, in contrast to that shown inFIGS. 1 and 4. Note that each inwardly extending surface is partlyconvex and partly concave, in order to minimize the covered area of eachwafer.

FIG. 5 illustrates boat 70 having three inwardly extending surfaces 74,each of which has a row of slots 75 equally spaced along thelongitudinal axis of the cylindrical shell. Note that vertical supportis provided only by the bottom support point, while each of the othersupport points is for lateral support or alignment only. In addition,the boat includes segments 76 and 77 which extend outwardly well beyondthe radius of the wafers, for the purpose of providing a stable base forthe boat.

Preferably at least three slot-containing inwardly convex segments, andmore preferably four slot-containing surfaces are provided in thehorizontal boats of the present invention. This is the case because:

(1) the bottom two slot-containing surfaces in the four slot-containingboat 20, or the single bottom slot-containing protrusion in the threeslot-containing boat 40 shown in FIG. 3, carry the weight of the wafer;and

(2) the top two slot-containing surfaces in boats 20 and 40 maintain theposition of the wafer in the vertical plane and ensure that each of thewafers is parallel to, and a uniform distance from each of the adjacentwafers.

The thickness of the CVD SiC, "t", should be minimized to reduce thethermal effects on the wafer, yet be thick enough to provide sufficientstrength. In the preferred embodiment, this thickness may be in therange of about 0.020 inch to about 0.15 inch or higher. Further t mayvary over the body of the boat, due to the nature of the CVD processand/or the requirements of the semiconductor manufacturing process. Forinstance, it is advantageous to have a thinner CVD SiC thickness at thesupport points to reduce the thermal effect of the boat on the wafers.

FIG. 6 shows a longitudinal section of equally spaced slots 35. Thisview is typical for each of slot-containing surfaces 34. Slots 35 areground or cut into the SiC, generally using a diamond grinding wheel bya process that is common and well known in the commercial grinding ofceramics. Note that if the slot depth, r4 minus r3, is greater than thethickness of the CVD SiC (t), the slots will be cut through thethickness of the CVD SiC as shown in FIGS. 1-5. The width of the slot,"C", is slightly greater than the thickness of the wafers to besupported. The tops of the slots are beveled at an angle, β, to helpguide the wafers into slots 35. The dimensions r3, r4, A, C, β, as wellas length of the boat and the distance from first end 25 to the firstslot 35 in each slot-containing protrusion 34 are defined by commonindustry standards such as the SEMI standards, by the manufacturers ofthe semiconductor processing equipment or by the manufacturer of thesemiconductor devices.

It may also be advantageous in some cases to vary the profile of theboat over its length. For instance, the preferred embodiment includes aprofile in which only the ends of the boat are flattened for use in thesupport of boat 20. For example, the flattened surface may be for adistance of approximately 1/2" measured longitudinally from first end 25and from second end 26 of boat 20. The center section between the firstand second ends of boat 20, comprising the greater portion of itslength, has the same curvature as cylindrical section 22.

A cross-section having such a profile is shown at 44 in FIG. 7. Thepreferred design of the carrier of the present invention provides theflattened surface at either end for support, positioning or lifting, aswell as the use of one or more cut outs for minimizing the mass andthermal effects.

While the previously cited references generally refer to hightemperature processing of semiconductor devices, the present inventionincludes the use of the CVD SiC component in operations that areperformed at lower temperatures, including room temperature or below.Many cleaning or etching processes take place at these lowertemperatures in corrosive or oxidative liquids or gases. In addition to,or in place of, elevated temperatures, these processes may useultrasonic, plasma or other processing techniques to produce the desiredeffect on the wafers. CVD SiC wafer boats of the present invention aremore stable in these corrosive environments.

Other embodiments of the present invention are contemplated, includingbut not limited to:

(a) a boat encompassing 1800 of a circle with closed ends, which mayinclude a corresponding cover to enclose the wafers;

(b) boats in which areas of the SiC have been indented, cut or groundaway to provide access for lifting devices, to reduce mass, to allow forfluid circulation or to allow for the draining of fluids in wetprocesses; and

(c) boats of contiguous design and non-contiguous design, as describedin the SEMI standards.

PROCESS FOR MAKING CVD SiC BOAT

FIGS. 7 and 8 illustrate that in the process of making one or morecarriers of the present invention, one or preferably multiple layers 54of SiC are deposited onto either cylindrical form 50 (FIG. 7) orcylindrical form 52 (FIG. 8). The desired geometric shape of the boat ismachined into forms 50 or 52 composed of graphite or other suitablesubstrate material for coating with CVD SiC. In the preferredembodiment, the graphite is purified using a high temperature chlorineprocess or other suitable purification process to minimize the contentof elements other carbon.

Referring to FIG. 7, mask 55 is placed over back 56 of form 50 toprevent the deposition of SiC. Similarly, the ends of form 50 (notshown) can also be masked to prevent closing the ends when an opened endboat is being made. Alternatively, these areas may be left unmasked, andthe subsequently coated surfaces can be ground or cut away to expose thegraphite form.

In FIG. 8, the geometric shape of the boat is machined onto both theright cylindrical section 58 and left cylindrical section 59 ofcylindrical form 52. A plurality of layers of SiC are deposited ontosections 58 and 59. Cylindrical form 52 is cut or ground longitudinallyalong the axes 60 and the graphite is removed to yield two boats. Inthis embodiment of the process of the present invention, the back of theboat is masked by the other boat around the circumference of thecylinder. If forms 50 and 52 are to be removed intact after coating, arelease agent may be applied to the outer surfaces of the form tofacilitate the separation of the form from the subsequently applied CVDSiC. The process illustrated by FIG. 8 may be varied so that more thantwo places around the circumference of cylinder 52 are machined in amanner so that more than two boats can be simultaneously manufactured.

Forms 50 and/or 52 are placed in a furnace suitable for applying a CVDSiC coating and a layer of CVD SiC is applied to the form using achemical vapor deposition process. Suitable processes for applying theCVD SiC coating are well known in the industry. The process generallyinvolves heating the form to a suitable temperature, introducing a gasor combination of gases which contain silicon and carbon atoms, thegases being at, above or below atmospheric pressure and allowing thegases to react to form a silicon carbide layer on the form. The SiClayer may be deposited in single or multiple steps to achieve thedesired thickness of silicon carbide. Examples of the suitable processesare described in the previously cited U.S. Pat. Nos. 3,962,391;4,093,201; 4,203,940; and 4,978,567 and Japanese Patent Publication JP50-90184.

The masks, if used, are removed to expose the underlying form or, ifmasks are not used, the CVD SiC coating is cut or ground away from theback and/or ends of the form. The graphite form is then removed intact,or removed by grinding, machining, burning, grit blasting, chemicallydissolving or oxidizing, or other suitable method or combination ofthese methods.

The resulting CVD SiC form is ground, using diamond grinding wheelsand/or other commercially available methods of shaping ceramics, to formthe slots, to reduce the form to the desired length and width, and toform the base and/or other features of the boat. In some instances itmay be advantageous to perform some or all of the grinding prior toremoval of the graphite form from the CVD SiC.

In some designs, it may be advantageous to grind holes completelythrough the CVD SiC, for instance, to provide open areas for gascirculation, for insertion of lifting devices to transport the boats,for the draining of fluids used in cleaning or wet processing, or forother reasons.

The method of the invention produces a boat having essentially thedesired final shape, upon completion of the deposition step. Thus,subsequent grinding is required for only 25% or less of the inner andouter surface areas. An especially unique feature of the process is theselective formation of relatively thinner walls in the areas of thecarrier where slots are provided to support the wafers.

EXAMPLE

A mold was prepared by machining a hollow graphite cylinder, to shapeits outer surface in the exact configuration required for the innersurfaces of two wafer carriers, respectively, one carrier to be formedon one side of the graphite cylinder, and the second carrier to beformed on the opposite side. Since the carriers are designed to holdwafers having a radius of 2.46", the graphite mold was selected to havea slightly larger outer radius of 2.72". Longitudinal concave grooveshaving a radius of 0.375" were machined into the graphite cylinder,positioned to provide the lower convex inner surfaces of the carriers,and another set of longitudinal concave grooves having a radius of0.514" were machined into the graphite cylinder, positioned to providethe upper convex inner surfaces of the carriers. The depth of each ofthe concave grooves machined into the graphite is controlled to provide0.01" of overlap on the edge of the wafers positioned in the slots to becut into the convex inner surfaces of the carriers.

The graphite cylinder was purified at 2,000 degrees C with chlorine gasin a purification reactor. The ends of the cylinder were then masked toprevent the coating gases from entering the interior of the cylinder.The masked cylinder was then placed in a CVD reactor and silicon carbidewas deposited on the exposed surfaces by the pyrolysis ofmethyltrichlorosilane. The CVD reactor was designed to rotate the partsto promote uniform coating. The deposition was completed in two separateruns, and the cylinder inverted after the first run. The masks wereremoved from the ends of the cylinder.

The cylinder was ground, using diamond tools, to the desired length of3.9375" and the bottom of the carrier forms ground flat at a distance of2.655" from the desired center of the wafers to be held. The graphitewas removed by combustion in air at 1600 degrees F. The wafer slots wereground into the carriers and the carriers separated from each other bygrinding with diamond tools. The slots were 0.1" deep, 0.035" wide andon 0.1875" centers.

The top portions of the slots are then beveled to facilitate insertionof the wafers, and exposed corners of the carrier are then chamferedusing a diamond grinding wheel.

Without departing from the spirit and scope of this invention, one ofordinary skill in the art can make many other changes and modificationsto the wafer carrier of the present invention to adapt it to specificusages and conditions. As such, these changes and modifications areproperly, equitably, and intended to be, within the full range ofequivalents of the following claims.

For example, when maximum boat strength and durability are required, thepreferred embodiment includes a combination of different profiles. Thatis, each end of the boat is configured to include inwardly convexsegments having slots therein to hold wafers; but not to include anysegments having a radius substantially greater than the radius of thewafer to be carried in the boat. Referring to FIG. 5, for example, eachend of such a boat includes segments 74 having slots 75; but does notinclude segments 76 and 77. The remainder of such a boat, except for theends, has the same cross-section as in FIG. 5. This still provides anadequate base for the boat, while minimizing its susceptibility tobreakage, because the ends normally receive more accidental impactstress from careless handling, than does the remainder of the boat.

What is claimed is:
 1. A process for making a carrier for holdingmultiple semiconductor wafers which comprises the steps of:a) machininga substrate material suitable for receiving a chemical vapor deposit ofsilicon carbide, into a cylindrical form, having a convex surfaceincluding at least two concave longitudinal surface segments; b) placingsaid form in a chemical vapor deposition reactor; c) chemical vapordepositing at least one layer of silicon carbide onto the form; d)separating the form from the silicon carbide layer, whereby theresulting silicon carbide carrier has the shape of a generallycylindrical shell with an average radius greater than the radius of thewafers to be held and that the inner concave surface of said shell hasat least two convex longitudinal segments extending inwardly; e)grinding into at least two of said convex surfaces a plurality of slotsfor supporting each of the wafers in a substantially vertical positionfor processing, and extending to an inward point such that a portion ofthe slot is within the diameter of the wafer to be held, and such thatthe distance from the inward point to the center of the wafer is lessthan the radius of the wafer; and f) recovering the resulting siliconcarbide carrier having an inner longitudinal surface of a diametergreater than the diameter of the wafers to be held.
 2. The process ofclaim 1 further including the step of grinding the silicon carbide layerto the desired shape of said carrier.
 3. A process for making a siliconcarbide carrier for holding semiconductor wafers, which comprises thesteps of:a) placing a suitable cylindrical substrate in a chemical vapordeposition reactor, said cylindrical substrate having a longitudinalsurface and first and second ends and having at least two of the desiredshapes of said carrier formed into the longitudinal surface of thecylindrical substrate along its longitudinal axis; b) chemical vapordepositing silicon carbide onto each of said shapes formed into thelongitudinal surface; c) cutting the cylindrical substrate apart alongits longitudinal axis; d) removing the cylindrical substrate materialfrom the silicon carbide; and e) recovering the resulting closed endsilicon carbide carriers, having the shape of a generally cylindricalshell having an average radius greater than the radius of the wafers tobe held and the inner concave surface of said shell having at least twoconvex longitudinal segments extending inwardly; and f) machiningorthogonal slots into the convex segments.
 4. The process of claim 3wherein the first and second ends of the cylinder are masked andopen-ended silicon carbide carriers are recovered.
 5. The process ofclaim 4 wherein, after the step of removing the cylindrical substrategrinding the silicon carbide layer to conform the surfaces of thecarrier to the desired shape.
 6. A process for making a carrier capableof holding multiple semiconductor wafers which comprises the steps of:a)machining a substrate material suitable for receiving a chemical vapordeposit of silicon carbide, into a form having the desired shape of thecarrier, including at least two concave longitudinal segments; b)placing said form in a chemical vapor deposition reactor; c) chemicalvapor depositing at least one layer of silicon carbide onto the form; d)separating the form from the silicon carbide layer, to recover a siliconcarbide carrier having the shape of a generally cylindrical shell withan average radius greater than the radius of the wafers to be held, andhaving an inner concave surface including at least two convexlongitudinal segments extending inwardly; and e) machining orthogonalslots into the convex segments, to produce a final carrier having slotsextending inward to a point within the diameter of the wafer to be heldand such that the distance from the inward point to the center of thewafer is less than the radius of the wafer.
 7. The process of claim 6wherein, before the step of removing the form, grinding the siliconcarbide layer to conform any exposed surface of the carrier to thedesired shape.
 8. The process of claim 6 wherein, after the step ofremoving the form, grinding the silicon carbide layer to conform anyremaining surface of the carrier to the desired shape.